Wind power generators have problems of noise generated from blades. The main causes of blade-generated noise are a blade tip vortex and a turbulent-flow boundary layer generated on the blade surface.
It is thought that decreasing the rotational speed of a rotor is effective in reducing the noise level. Specifically, decreasing the rotational speed of the rotor decreases the inlet velocity of air to the blades, thereby allowing aerodynamic sound to be reduced. However, decreasing the rotational speed of the rotor has a problem of decreasing the power generation efficiency.
In particular, for reducing noise at a high-noise blade tip portion, for example, an approach disclosed in Patent Citation 1 has been proposed.
This prevents noise by curving or bending the blade tip portion on the blade surface toward the trailing edge of the blade (a so-called sweptback wing) and/or providing a small blade (a so-called winglet) that is bent toward a positive pressure surface so as to intersect the blade tip portion to thereby decrease the vortex at the blade tip portion.
Patent Citation 1: Japanese Translation of PCT International Application, Publication No. 2006-521483
In recent years, wind power generators have been required to generate high power and have increased in size. Along with this, the rotor diameter, that is, the blade lengths of wind turbine blades, have been increased.
With the increase in blade length, the moving speed of blade tip portions increases. This increases noise generated, and therefore, a further noise reduction is required.
Moreover, because the influence of noise due to turbulent-flow boundary layers generated on blade surfaces in the vicinity of the blade tips is increased due to the increase in blade length, sufficient noise reduction cannot be expected with an approach that reduces noise only at the blade tip portions, as in Patent Citation 1.
The present invention is made in consideration of the circumstances described above, and it is an object thereof to provide a wind turbine blade that can reduce the thickness of the turbulent-flow boundary layer generated on the blade surface at the blade tip portion to prevent noise generated at the trailing edge portion effectively, as well as a wind power generator using the same.
The present invention adopts the following solutions to solve the problems described above.
According to a first aspect of the present invention, there is provided a wind turbine blade having an airfoil shape in cross section, wherein an angle-of-attack decreasing portion that decreases an angle of attack during rotation at at least a predetermined rotational speed is provided over a predetermined range from a blade tip to a blade root side.
With the wind turbine blade according to this aspect, since the angle-of-attack decreasing portion decreases the angle of attack during rotation at at least the predetermined rotational speed over a predetermined range from the blade tip to the blade root side, the thickness of the boundary layer on the blade surface of the predetermined range from the blade tip to the blade root side can be decreased during operation.
The decrease in thickness of the boundary layer can prevent the occurrence of a vortex at the trailing edge of the blade, thus reducing noise.
Since the angle-of-attack decreasing portion is provided at the tip portion of the wind turbine blade at which high noise is generated, noise can be reduced effectively. This eliminates the need for decreasing the rotational speed to reduce the noise, which can prevent a degradation in performance, such as a degradation in power generation efficiency.
Given the same cross-sectional shape of the blade, the lift generated is decreased with a decrease in angle of attack; therefore, it is preferable to set the range in which the angle-of-attack decreasing portion is provided in consideration of the influence on the performance of the blade and the noise reduction effect. This range is set, for example, within 20% of the blade length, preferably, within 10%.
The predetermined rotational speed is set to, for example, a rotational speed at which the wind power generator outputs rated power, that is, a rated rotational speed.
In the above aspect, the angle-of-attack decreasing portion may be formed so that the angle of attack gradually decreases from the angle of attack of the blade root side toward the blade tip.
In other words, the angle-of-attack decreasing portion is formed such that it is twisted so that the angle of the blade with respect to the wind inflow direction decreases gradually toward the blade tip, that is, a twist angle is applied to the blade tip portion.
This makes the angle of attack of the angle-of-attack decreasing portion always differ from the blade root side irrespective of the rotational speed, which can reduce noise in a wide rotational speed range.
In the above aspect, the angle-of-attack decreasing portion may be formed so as to become substantially an extension of the blade surface of the blade root side during rotation at the predetermined rotational speed.
In other words, the surface of the angle-of-attack decreasing portion is formed to become substantially flush with the blade surface of the blade root side during rotation at the predetermined rotational speed.
When a blade rotates, the blade is deformed due to wind. This deformation becomes large at the tip portion of the blade. This deformation makes the angle of attack at the tip portion of the wind turbine blade larger than a design value, thus increasing noise.
With this configuration, when the blade rotates at the predetermined rotational speed and is deformed due to wind, the angle-of-attack decreasing portion is formed so as to become substantially an extension of the blade surface of the blade root side, that is, such that the blade surface of the blade root side becomes flush with the blade surface of the angle-of-attack decreasing portion, which therefore prevents the angle of attack of the angle-of-attack decreasing portion from becoming larger than the design value; in other words, it can decrease the angle of attack at the predetermined rotational speed as compared with a blade having no angle-of-attack decreasing portion.
This can reduce noise at the predetermined rotational speed and can prevent a degradation in performance
With the above configuration, it is preferable that the angle-of-attack decreasing portion be smoothly curved so that the curve gradually becomes sharp from the blade surface of the blade root side toward a positive pressure surface side.
According to a second aspect of the present invention, there is provided a wind power generator that generates electricity using a wind turbine blade in which the occurrence of a vortex at the trailing edge of the blade during rotation at at least a predetermined rotational speed can be prevented.
This allows the wind power generator to reduce the occurrence of noise during operation. Since this can relax restrictions due to noise, the flexibility of the installation site can be ensured; for example, it can be installed in the vicinity of a residential district.
According to the present invention, since the angle-of-attack decreasing portion decreases the angle of attack during rotation at at least the predetermined rotational speed over the predetermined range from the blade tip to the blade root side, the occurrence of a vortex at the trailing edge of the blade can be prevented during operation, thus reducing noise.
Moreover, since the angle-of-attack decreasing portion is provided at the tip portion of the wind turbine blade at which high noise is generated, noise can be reduced effectively. This eliminates the need for decreasing the rotational speed to reduce the noise, which can prevent a degradation in performance, such as a degradation in power generation efficiency.